The drug discovery process is currently undergoing a fundamental revolution as it embraces `functional genomics`, that is, high throughput genome- or gene-based biology. This approach is rapidly superseding earlier approaches based on `positional cloning`. A phenotype, that is a biological function or genetic disease, would be identified and this would then be tracked back to the responsible gene, based on its genetic map position.
Functional genomics relies heavily on the various tools of bioinformatics to identify gene sequences of potential interest from the many molecular biology databases now available. There is a continuing need to identify and characterise further genes and their related polypeptides/proteins, as targets for drug discovery.
Protein phosphorylation plays a critical role in promoting cell cycle progression. Most prominent among the regulators of the cell cycle is a family of cyclins, cyclin dependent kinases (CDKs), CDK regulatory kinases, and phosphatases (See Lees, E., Curr. Opin. Cell Biol. 1995, 7:773-780; Piwinica-Worms, H., J. Lab. Clin. Med. 1996, 128:350-354). A new family of cell cycle regulators, the polo-like kinases, has been identified and shown to be essential for progression through the cell cycle (Lane, H. A., Trends in Cell Biol. 1997, 7:63-68). This subfamily of serine/threonine kinases contains the following related but distinctmembers: (1) Plk (polo-like kinase; human) and its homologs Polo (Drosophila), cdc5 (S. cerevisiae), Plx (Xenopus), and Plo (S. pombe); (2) Prk (polo-related kinase; human) and its murine homologFnk; and (3) Snk (serum-inducible kinase; murine). Known functions of these genes include regulation of spindle assembly (human plk1, Drosophila polo, S. pombe plo1) and late nuclear division (S. cerevisiae cdc5). PLK1 expression correlates with the mitotic index (Holtrich U., Proc. Natl. Acad. Sci. 1994, 91:1736-1740) and mutations of the Drosophila polo or S. cerevisiae cdc5 gene cause mitotic arrest. In addition, antibodies directed against human PLK1 cause impaired mitosis. Progression from the G2 phase to the M phase of the cell cycle requires the activity of cdc25 phosphatase. PLX1 (Xenopus) phosphorylates and, thereby, activates cdc25-c, an isoform of cdc25 (Dunphy W. G., Science 1996. 273:1377-1380). The murine Snk is an early growth response gene which reportedly phosphorylates heterologous (although unidentified) substrates (Simmons D. L., Mol. Cell.Biol. 1992, 12:4164-4169). Identification of the consensus sequence of the polo-like family in the amino-terminal putative catalytic domain of Snk (published murine sequence and present invention) and the consensus polo box sequence in the carboxy terminus place this protein in the polo-like family and suggest that this enzyme is potentially a critical regulator of cell cycle progression.